There is a belt-type CVT (Continuously Variable Transmission) capable of changing the gear ratio of a transmission of a vehicle in a stepless manner according to the traveling state of the vehicle (for example, Patent Literature 1). FIG. 13 is a perspective view showing an endless metal belt 10 used for the CVT. FIG. 14 is a partial perspective view showing a part of the endless metal belt 10 in which the endless metal belt 10 is partially disassembled.
The endless metal belt 10 is obtained by concatenating a plurality of metal elements 11 each of which is a plate-like element in the thickness direction and forming the concatenated metal elements 11 into a ring shape as a whole. To support the plurality of metal elements 11 in the ring shape, multi-layered rings 12 are inserted into the metal elements 11 from both sides thereof in the width direction.
Each of the multi-layered rings 12 is formed by stacking six to twelve endless metal rings 13 having slightly different circumferential lengths on each other. (In FIG. 13, only three endless metal rings are layered because the structure is simplified for clarifying the illustration.)
For the purpose of explaining, the inner most endless metal ring 13 is referred to as a “first ring” and the endless metal ring 13 layered on the outer side of the first ring is referred to as a “second ring”. Further, other metal rings 13 are named in such a manner that their numbers increase from the inner side toward the outer side of the rings. The circumferential length of a ring is slightly longer than that of a ring disposed immediately inside that ring. Therefore, when the outer-side ring is layered on the inner-side ring, these rings are substantially in intimate contact with each other throughout the entire circumference.
To manufacture the multi-layered ring 12, it is necessary to control the circumferential length of each endless metal ring 13 in an extremely strict manner so that the clearance between an inner ring and the ring disposed immediately outside that ring (hereinafter called an “inter-layer clearance”) is smaller than a prescribed tolerance.
For example, when the inter-layer clearance between the k-th ring and the (k+1)-th ring is larger than the prescribed tolerance, it causes these rings to be rubbed against each other and thus eventually to be torn.
Further, six to twelve rings have to be successively layered on each other. Therefore, if the inter-layer clearance between the k-th ring and the (k+1)-th ring is larger than the prescribed tolerance, it makes it impossible to layer the (k+2)-th ring or the (k−1)-th ring outside or inside them, thus eventually making it impossible to assemble them into a multi-layered ring.
Needless to say, if there is no inter-layer clearance at all or if, when taking the plate thickness into consideration, the outer circumferential length of the k-th ring is larger than the inner circumferential length of the (k+1)-th ring, they cannot be layered on each other as a matter of course.
Therefore, the circumferential lengths of rings are adjusted so that these rings exhibit circumferential lengths respectively predetermined for them. That is, each ring is stretched to its determined circumferential length by applying a tension (tensile force). Note that there are predetermined target values for the circumferential lengths for the rings, i.e., the ring that will become the first ring, the ring that will become the second ring, and so on. For this process in which a ring is stretched by applying a tension (tensile force), various technical ideas have been proposed to adjust the circumferential length with a very high accuracy.
For example, Japanese Unexamined Patent Application Publications No. 2011-185300 (Patent Literature 2) and No. 2013-52432 (Patent Literature 3), which were filed by the applicant of the present application, disclose circumferential length adjustment devices and circumferential length adjustment processes in which considerable contrivances are made.
Meanwhile, a multi-layered ring, i.e., each of the endless metal belts constituting the multi-layered ring, needs to exhibit a high strength as a matter of course. Therefore, after the circumferential length adjustment, a heat treatment(s) such as aging, oxidizing, and nitriding is performed in order to improve, for example, the hardness and/or the wear resistance of the multi-layered ring (Patent Literatures 4 and 5).